1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device, and more particularly, to a trans-reflective type LCD device and a method for fabricating the same, using a half-tone mask and a diffraction exposure.
2. Discussion of the Related Art
In general, LCD devices are classified into a transmitting type LCD device using a backlight as a light source, and a reflective type LCD device using ambient and artificial light as a light source without using a backlight. Since the transmitting type LCD device uses the backlight as the light source, it can display images in dark surroundings. However, the transmitting type LCD device has the disadvantageous characteristics in that it cannot be used in the bright surroundings, and it has high power consumption. The reflective type LCD device does not use a backlight and thus consumes less power. However, the reflective type LCD device cannot be used in the dark surroundings.
A related art solution is a trans-reflective type LCD device. The trans-reflective type LCD device includes unit pixel regions, each unit pixel having a transmitting part and a reflective part, whereby it can use both ambient light and the light generated from a backlight. Thus, with a trans-reflective type LCD device, it is possible to decrease power consumption and use the device in various ambient light conditions.
The related art LCD device has an additional storage capacitor to support the charge maintenance capacity of liquid crystal. The structure forming a capacitor between a gate line and a pixel electrode is referred to as a storage on gate structure, and the structure forming a capacitor between a common electrode line and a pixel electrode is referred to as a storage on common structure. The storage capacitor helps maintain a voltage applied to a pixel electrode by a corresponding thin film transistor. Accordingly, the storage capacitor mitigates current leakage between subsequent pixel voltage applications and thus helps prevent deterioration of picture quality induced by flicker.
FIGS. 1-3 illustrate a trans-reflective LCD according to the related art.
FIG. 1 is an exploded perspective view of some parts of a general trans-reflective type LCD device. As illustrated in FIG. 1, the general trans-reflective type LCD device 11 includes an upper substrate 15, a lower substrate 21, and a liquid crystal layer 23. The upper substrate 15 includes a color filter 17 including a black matrix 16, and a transparent common electrode 13 formed on the color filter 17. The lower substrate 21 includes a pixel electrode 19 having a transmitting part A and a reflective part C in a pixel region, a switching device T, and an array line. The liquid crystal layer 23 is formed between the upper substrate 15 and the lower substrate 21.
The lower substrate 21 is referred to as a TFT array substrate, on which a plurality of gate lines 25 are formed perpendicularly to a plurality of data lines 27, defining a plurality of pixel regions. A plurality of thin film transistors T are formed at respective crossing portions of the plurality of gate and data lines 25 and 27, wherein the plurality of thin film transistors T are formed in a matrix-type configuration.
An operation of the general trans-reflective type LCD device will be described with reference to FIG. 2.
FIG. 2 is a cross sectional view of the general trans-reflective type LCD device. As illustrated in FIG. 2, the trans-reflective type LCD device 11 includes the upper substrate 15, the lower substrate 21, the liquid crystal layer 23, and a backlight 41. The upper substrate 15 has the common electrode 13, and the lower substrate 21 has the pixel electrode 19 including a transmitting electrode 19a formed in the pixel region P that includes transmitting part A, and a reflective electrode 19b formed in the pixel region P that excludes transmitting part A. Also, the liquid crystal layer 23 is formed between the upper substrate 15 and the lower substrate 21, and the backlight 41 is provided below the lower substrate 21.
If the trans-reflective type LCD device 11 is operated in a reflective mode, the trans-reflective type LCD device 11 uses ambient light.
An operation of the trans-reflective type LCD device in transmitting mode and reflective mode will be described as follows.
In reflective mode, the trans-reflective type LCD device uses ambient light. That is, light B, which is incident on the upper substrate 15 of the trans-reflective type LCD device 11, is reflected off on the reflective electrode 19b. The reflected light passes through the liquid crystal layer 23 aligned by an electric field between the reflective electrode and the common electrode 13, and the amount of light B passing through the liquid crystal layer 23 is controlled according to the alignment of the liquid crystal molecules within liquid crystal layer 23, thereby displaying the image.
In the transmitting mode, the trans-reflective type LCD device uses the light F emitted from the backlight 41 below the lower substrate 21. That is, the light F emitted from the backlight 41 is incident on the liquid crystal layer 23 through the transmitting electrode 19 and the transmitting part A. The amount of light transmitted from the backlight 41 through the LCD structure is controlled according to the alignment of the liquid crystal molecules within liquid crystal layer 23, thereby displaying an image. The alignment of the liquid crystal molecules is controlled by an electric field between the transmitting electrode 19a and the common electrode 13. The electric field corresponds to the voltage applied to the pixel electrode by the thin film transistor.
Generally, the LCD device includes a thin film transistor array substrate referred to as a lower substrate, a color filter substrate referred to as an upper substrate, and a liquid crystal layer formed between the lower and upper substrates.
A method for fabricating the trans-reflective type LCD device according to the related art is described as follows.
FIG. 3 is a cross sectional view of the trans-reflective type LCD device according to the related art. The trans-reflective type LCD device of FIG. 3 is fabricated with nine masks according to the following procedure.
First, a buffer insulating layer 51 is formed on a substrate 50. After that, an amorphous silicon layer is deposited on the substrate, and is crystallized to a polysilicon layer by a thermal curing process and a laser curing process.
Then, the polysilicon layer is patterned by photolithography using a first mask, thereby forming a semiconductor pattern 52 on portions corresponding to a thin film transistor and a storage capacitor. Subsequently, a gate insulating layer 53 and a conductive metal layer are sequentially deposited on an entire surface of the substrate 50 including the semiconductor pattern 52. The conductive metal layer is then selectively removed by photolithography using a second mask, thereby forming a gate line (not shown) and a gate electrode 54a projected from the gate line. A common line (not shown) is formed in parallel with the gate line, and is overlapped with the semiconductor pattern 52. A portion of the common line overlapped with the semiconductor pattern 52 forms a storage electrode 54b. 
Next, n-type or p-type impurity ions are implanted into the semiconductor pattern 52 by using the gate electrode 54a as a mask, thereby forming source and drain regions 52a and 52b, respectively. A first insulating interlayer 55 is then formed at a thickness of 7000 Å on the entire surface of the substrate 50 including the gate electrode 54a. The first insulating interlayer 55 and the gate insulating layer 53 are patterned by photolithography using a third mask, thereby forming first and second contact holes in the source/drain regions 52a/52b. 
A conductive metal layer is deposited on the entire surface of the substrate including the first and second contact holes, and is patterned by photolithography using a fourth mask to form the source electrode 56a that is electrically connected to source region 52a, and drain electrode 56b that is electrically connected to drain region 52b. 
Next, second and third insulating interlayers 57 and 58 are deposited in sequence, wherein the second and third insulating interlayers 57 and 58 are formed of silicon nitride and BCB (benzocyclobutene). The third insulating interlayer 58 is formed as an uneven surface by using a fifth mask. Subsequently, the second and third insulating interlayers 57 and 58 are etched by photolithography using a sixth mask, thereby forming a first hole in the transmitting part, and a third contact hole on the drain electrode 56b. 
A reflective electrode 59 is formed by depositing a reflective metal layer on the entire surface of the substrate to be in contact with the drain electrode 56b through the third contact hole, and patterned by photolithography using a seventh mask. Then, a fourth insulating interlayer 60 of silicon nitride is deposited on the entire surface of the substrate 50. The fourth insulating interlayer is etched by photolithography using an eighth mask, thereby exposing a predetermined portion of the reflective electrode 59 and the transmitting part A.
Subsequently, a transparent conductive layer is deposited on the entire surface of the substrate, to be in contact with the reflective electrode 59, and then is patterned by photolithography using a ninth mask, thereby forming a transmitting electrode 61 in a pixel region. The storage capacitor is formed by the semiconductor pattern 52, the gate insulating layer 53, and the storage electrode 54b. 
As explained above, the trans-reflective type LCD device according to the related art is fabricated with the nine masks. The fabrication steps are complicated due to the alignment process of the mask and the exposure and development process, thereby lowering the yield.
Also, it is generally required to perform the additional thermal process for forming the uneven surface of the reflective part, which may deteriorate materials that would otherwise be desirable for the insulating material under the reflective electrode.
In addition, the transmissive electrode of the related art is generally formed on top of the reflective electrode, which generally decreases the reflective area of the reflective electrode.